The use is known of so-called workshop tyre-changing machines which allow fitting and removing tyres onto and off the relative rim of a vehicle wheel, e.g., for carrying out maintenance jobs or replacing the rim and/or the tyre itself.
Such tyre-changing machines generally consist of a base structure supporting means for gripping and placing in rotation the rim of a wheel, having a rim locking spindle, and of at least a tool bearing arm having one or more tools suitable for removing and/or fitting the tyre from and onto the rim.
Different types of means for gripping and placing in rotation the rim of a wheel exist, made up of different types of rim locking spindles.
A particular type of means for gripping and placing in rotation the rim is made up of gripping means for gripping the edge of the rim of the wheel.
These gripping means for gripping the edge of the rim can be composed of, e.g., a so-called spindle with jaw lock.
In particular the spindle with jaw lock consists of a fastening plate for the rim, which is fitted on the base structure of the tyre-changing machine in a rotatable way around a central work axis and which has four gripping jaws of the edge of the rim.
The jaws are fitted on respective supporting slides and move, due to the action of suitable actuators, from the centre towards the outside of the plate and vice versa, between a closing configuration, corresponding to the positioning of the jaws at the centre of the plate, and an opening configuration, corresponding to the positioning of the jaws at the periphery of the plate.
In particular, the jaws can be used so as to engage on the edge of the rim both from the outside and the inside.
Once moved to the grip position, the jaws are engaged in correspondence to respective sections of the edge of the rim, so as to lock the rim itself onto the plate.
The need is also known to make a further adjustment of the position of the jaws, irrespective of the adjustment commonly made by means of the actuators, so as to also allow the use of the spindle for rims of different dimensions.
In particular, the use is increasingly more common and widespread of rims with greater diameter compared to the rims of conventional type. Consequently, conventional type jaw spindles cannot be used because the maximum diameter achievable with the actuators fully extended and the jaws in maximum-opening position is less than the diameter of such rims.
To overcome this problem, the manual adjustment is known of each of the jaws, done by moving and suitably positioning each of the jaws on the respective supporting slides.
Such manual operation nevertheless is often complicated, costly in terms of the time required and not always effective, inasmuch as the operator who has to perform the adjustment can make mistakes, and thereby negatively affect the correct operation of the spindle.
To overcome such drawbacks, the document EP 1 518 718 is known, wherein is described a self-centring gripping device for tyre-changing machines comprising a supporting plate having a plurality of radial slats, equidistant from one another, inside which are inserted sliding relative gripping jaws of a rim.
The jaws are connected to one another by means of a system of levers, so they are always equally distant with respect to the rotation axis of the plate, and are associated with actuator means suitable for causing their translation in a radial direction.
Furthermore, between at least one of the jaws and the actuator means is placed a positioning device suitable for changing the work position of the jaws, with respect to the actuator, without changing its stroke.
In particular, the positioning device comprises a crankshaft with a crank, the button of which is suitable for being received by a bush integral with one of the jaws, and the outer pins of which are connected to operating means.
Special locking means are suitable for locking the crankshaft in two opposite contrasting work positions.
Nevertheless, such self-centring gripping device is not without its drawbacks either.
In particular, the positioning device used is structurally complex and its realization has a significant influence on the manufacturing times and costs of the self-centring gripping spindle of the tyre-changing machine.
Another drawback is represented by the fact that the positioning device used only permits positioning the jaws in two extreme positions and does not therefore permit making a precise adjustment of the position of the gripping jaws in accordance with the specific dimensions of the rim to be locked.
Furthermore, the possibility of only positioning the jaws in two extreme positions makes necessary continuous adjustment operations including in the event of its being necessary to operate in sequence on the rims with different dimensions the one from the other but which, in any case do not have extreme dimensions.
This therefore means a considerable increase in total tyre fitting and/or removal times.